The present invention relates to a control circuit for a glow plug assembly serving as an engine preheating means, and more particularly to a control circuit designed so as to effect a temperature control in harmony with the specified position established by selective operation of a key switch.
Generally, before starting an engine with a starter motor, it is necessary to set a temperature of the glow plug assembly, serving as an engine preheating temperature, to the level suitable for immediate starting thereof, for example, substantially 800.degree. C. In the prior art, power is applied to the glow plug assembly in accordance with the operation of a key switch to heat the glow plug assembly. However, with this method, the following drawback is pointed out; there is a possibility that the temperature of the glow plug assembly excessively increases or decreases. As a result, the durability of the glow plug assembly is reduced or the engine cannot be started for a short interval.
Reference is made to another aspect of the preheating method. For instance, in the event that the temperature of the engine is relatively high, the engine can be sufficiently started by preheating the glow plug assembly simultaneously with the selection of the key switch to the START position. However, in prior arts, the glow plug assembly is designed so as to be preheated solely in accordance with the operation of the key switch to the ON position. Therefore, the prior art device is not favorable in terms of power consumption.